This application proposes to support the career development of Alan H. Beggs, Ph.D., an Assistant Professor of Pediatrics at Children's Hospital and Harvard Medical School. The applicant's career goals are to continue developing a vigorous independent research program in muscle biology and human neuromuscular disease in the rich and stimulating environment of the Boston biomedical research community. Salary support via the KO2 mechanism will allow him to pursue full time research without having to perform a service function such as running a clinical diagnostic laboratory. The scientific goals of this proposal are to understand the structures, functions and protein interactions of muscle-specific alpha- actinins and to apply this information to the study of human neuromuscular disorders. The alpha-actinins are a family of closely related actin-binding proteins that serve to cross link and anchor actin filaments. In muscle, calcium-insensitive alpha-actinins are a major component of Z lines where they constitutively anchor the actin/nebulin-containing thin filaments. A number of human neuromuscular diseases have been shown to result from mutations of muscle-specific cytoskeletal elements. Similarly, several inherited cardiomyopathies are caused by mutations in genes for cardiac- specific isoforms of sarcomeric proteins. It is hypothesized that some human neuromuscular diseases may be caused by mutations in muscle-specific alpha-actinin genes, and/or in genes for proteins that interact with alpha-actinin. The applicant has cloned and characterized three genes for human o:-actinins and has recently identified several patients with congenital muscular dystrophy who lack expression of one of the muscle-specific isoforms. This proposal entails: l) extending this observation in additional patients with primary disorders of muscle and identifying mutations in the o(-actinin genes of deficient patients; 2) further characterizing (L-actinins in normal tissues; 3) developing cell culture and transgenic mouse models of alpha-actinin dysfunction; 4) identifying genes for novel proteins that interact with alpha- actinins; 5) characterizing these new genes and proteins: and 6) assessing these as candidate genes for other human neuromuscular diseases. Direct clinical benefits will include accurate pre- and postnatal diagnoses for these disorders, better understanding of their underlying etiology, and insights into potential therapies. These experiments will also increase our understanding of a-actinin function and identify new interactions with existing and novel muscle proteins at the Z-line and elsewhere.